1. Field of the Invention
The present invention relates to a method of cleaning an MOCVD (Metal Organic Chemical Vapor Deposition) apparatus or another CVD apparatus having a vaporizer for vaporizing a liquid raw material by applying heat and a processing chamber for forming a thin film on a substrate by a CVD (Chemical Vapor Deposition) method using the raw material vaporized in the vaporizer. More specifically, the present invention relates to a method of removing residues in the vaporizer and the processing chamber.
2. Description of the Related Art
In general, a liquid material, which is liquid at room temperatures, is generally sent in a vaporized state by a method disclosed in Japanese Patent Laid-Open No. 50-211072 and using a so-called bubbler or a bubbling apparatus. Also in a process for manufacturing semiconductors, the above-mentioned method is employed in a multiplicity of film forming apparatuses including those for forming TEOS (tetraethoxyorthosilane) films or those for forming superconductive thin films.
On the other hand, dielectric thin films have attracted attention in recent years as an important technique for manufacturing next-generation DRAM (Dynamic Random Access Memory) and FRAM (Ferroelectric Randam Access Memory) apparatuses, the dielectric or ferroelectric thin films including, for example, BST (BaSrTiO.sub.3, that is, barium strontium titanate) films, SrTiO.sub.3 (strontium titanate) films and PZT (PbZrTIO.sub.3) films.
When a dielectric thin film is formed by a CVD apparatus, a metal organic material, such as Ba(DPM).sub.2, Sr(DPM).sub.2 or Pb(DPM).sub.2, is employed (DPM: dipivaloyl methane). Since each of the above-mentioned materials is solid in room temperatures, the material must be maintained at a hot state not lower than about 200.degree. C. and thus maintained in a liquid state when the material in a gas state is sent by the bubbling apparatus. However, a fact is known that the raw material is quickly decomposed and allowed to deteriorate in the above-mentioned hot state.
To realize a liquid state at relatively low temperatures, a method has been developed with which the metal organic material (a solid raw material) is dissolved in an adduct (a type of a solvent) such as THF (tetrahydrofuran). If the vaporized material is sent by the bubbling apparatus, the pipe arranged from the bubbling apparatus to the processing chamber must be maintained at 200.degree. C. or higher to prevent condensation and liquefaction of the material. Therefore, the hot pipe state must be maintained. In this case, there arises a problem in that only the adduct, such as the THF, is decomposed and vaporized and thus a required material, such as Sr(DPM).sub.2, is condensed and fixed to the inner portion of the pipe and the like.
To solve the above-mentioned problem, research of a method has begun recently, with which a liquid raw material obtained by dissolving the above-mentioned required solid raw material in a solvent is, in the liquid state, sent, and then the raw material is heated in a vaporizer formed at the front of the processing chamber so as to be vaporized and immediately supplied to the inside portion of the processing chamber.
A CVD apparatus having the above-mentioned vaporizer has been disclosed in Unexamined Japanese Patent Publication (kokai) No. Hei 7-268634. An example of the CVD apparatus is shown in FIG. 4.
A liquid raw material 4 obtained by dissolving a required solid raw material, such as Sr(DPM)2, in a solvent, such as THF, is supplied from a liquid raw-material supply apparatus 2 to a vaporizer 8 through a liquid raw-material pipe 6 in a predetermined quantity.
The liquid raw-material supply apparatus 2 according to this example has a liquid raw-material container 42 for accommodating the liquid raw material 4, valves 44 to 47, a flow-rate adjustment unit 48 and pipes for connecting the above-mentioned elements. The liquid raw-material container 42 is, through a valve 44, supplied with an inert gas 50 for sending the liquid raw material 4 with pressure. The inert gas 50 is composed of at least a nitrogen gas or a rare gas (that is, He, Ne, Ar, Kr, Xe or Rn which is applied hereinafter). When the liquid raw material 4 is sent under pressure, the valves 44, 45 and 47 are opened and the valve 46 is closed.
The vaporizer 8 has a structure in which a gas introducing pipe 16 is connected to a vaporizing container 10. Moreover, a nozzle 14 is, coaxially with the gas introducing pipe 16, inserted into the vaporizing container 10. In addition, a heater 12 is disposed to surround the vaporizing container 10. The nozzle 14 is connected to the liquid raw-material pipe 6. The gas introducing pipe 16 is supplied with an inert gas 18 through a flow-rate adjustment unit 17. Also the inert gas 18 is composed of at least either of the nitrogen gas or the rare gas.
The liquid raw material 4 supplied to the vaporizer 8 is, at the leading end of the nozzle 14, roughly particulated by the high speed inert gas 18 flowing around the leading end. Thus, the liquid raw material 4 is dispersed and allowed to collide with a wide range of the inner wall of the vaporizing container 10 heated to temperatures not lower than 250.degree. C. so that the liquid raw material 4 is immediately vaporized. A vaporized raw material 20 is allowed to pass through a vaporized raw-material pipe 22 and a valve 24, and then supplied to the inside portion of the processing chamber 26.
In the processing chamber 26, a holder (also called a susceptor) 36 for holding a substrate 34 on which a film will be formed and a gas diffusing plate 32 having a multiplicity of small openings and arranged to diffuse a gas introduced into the processing chamber 26 are disposed. The holder 36 and the substrate 34 disposed above the holder 36 are heated by a heating means (not shown). A vacuum exhausting unit 40 for vacuum-exhausting the inside portion of the processing chamber 26 is connected to the processing chamber 26 through a valve 38. In addition to the vaporized raw material 20, a gas 30 arranged to react with the vaporized raw material 20 is introduced into the processing chamber 26. When a thin film made of SrTiO3 is formed, the gas 30 is a mixed gas of TTIP {Ti (O-i-C.sub.3 H.sub.7) and an oxide gas (O.sub.2 or the like). The vaporized raw material 20 and the gas 30 are mixed in the processing chamber 26. The mixed gas is dispersed to have a uniform flow velocity by the gas diffusing plate 32, and then diffused in the processing chamber 26 vacuum-exhausted by the vacuum exhausting unit 40. Then, the mixed gas is brought into contact with the heated surface of the substrate 34. As a result of CVD reactions, a thin film made of SrTiO.sub.3 or the like is formed. The mixed gas which has not been used to form the thin film is discharged to the outside through the vacuum exhausting unit 40.
The above-mentioned raw material, such as Sr(DPM).sub.2 or Ba(DPM).sub.2, is easily bonded to trace impurities, such as H.sub.2 O, CO, CO.sub.2 or the like, and precipitated. If the environmental temperature is high, the raw material is gradually decomposed and precipitated because of variation with time. Residues of the raw material are accumulated in the vaporizer 8 (specifically, the vaporizing container 10 of the vaporizer 8 which applies hereinafter), causing a variety of problems to arise. For example, residues are fixed to the inner wall of the vaporizing container 10, causing the efficiency to vaporize the liquid raw material 4 to deteriorate or the state of fixation to become nonuniform. As a result, the vaporization becomes instable. Moreover, residues sometimes cause the nozzle 14 to be clogged. If fixed residues are separated, there is apprehension that the downstream valve or the pipe is clogged.
To prevent this, a cleaning solution (for example, nitric acid) capable of dissolving residues is used to periodically clean the inside portion of the vaporizing container 10. The exhaust gas 21 generated after the cleaning process has been performed is allowed to pass through the vaporized raw-material pipe 22 and a valve 52, and then discharged to the outside by a vacuum exhausting unit 54.
However, residues are accumulated in the processing chamber 26 (specifically, the inner wall of the processing chamber 26 and the surfaces of the gas diffusing plate 32 and the holder 36) as well as in the vaporizer 8. Separation or the like of residues results in generation of particles (dust) which adhere to the surface of the substrate 34. In this case, there arises a problem of contamination of the surface of the substrate and the like arises.
Therefore, the cleaning solution for dissolving residues has been used to sometimes clean the inside portion of the processing chamber 26. The process for cleaning the inside portion of the processing chamber 26 and that for cleaning the inside portion of the vaporizer 8 have been performed independently. Moreover, fixed residues cannot easily be removed by the cleaning solution and thus a long time is required to remove fixed residues. As a result, an excessively long time is required to complete the cleaning process. Thus, time, for which the operation of the CVD apparatus is interrupted, is elongated excessively, causing the throughput of the CVD apparatus (processing performance per unit time period) to deteriorate.